Gasoline engines are equipped with three-way catalytic converters to oxidize CO and HC engine emissions and reduce NOx emissions. An oxygen sensor and feedback control can be located in the exhaust before the catalyst (referred to as inner fuel loop control), however this part of the system is not the subject of this invention. Rather, a feedback air/fuel control responsive to a narrow band exhaust gas oxygen sensor, located after the catalyst, is commonly used to maintain on average a stoichiometric air/fuel ratio, forming what is known as an outer fuel loop control. When exhaust gasses are lean of stoichiometry (e.g. excess oxygen is detected by the sensor), the sensor will provide a lower voltage and the feedback controller will enrich the air/fuel charge delivered to the engine (either via an open loop command to the injectors or through a command to the inner loop control). If the exhaust gasses are rich of stoichiometry the sensor outputs a higher voltage and the feedback controller correspondingly provides a leaner air/fuel charge. The set point of the sensor output is set at a voltage corresponding approximately to stoichiometry.
During the lean air/fuel transients resulting from feedback control, excess oxygen will oxidize CO and HC in the exhaust stream over the precious metal catalyst surfaces and O2 also will be stored in ceria compounds in the catalyst. During the rich transients, HC and CO will reduce NOx in the exhaust stream over precious metal catalyst surfaces and also reduce cerium oxide that is used as O2 storage. In this way the air/fuel ratio is essentially maintained at stoichiometry. CO and HC emissions will be oxidized and NOx emissions reduced in compliance with government regulations.
Emission control is complicated subsequent to a deceleration event during which fuel is cutoff to the engine cylinders and air is pumped through the cylinders (DFSO). The pumped air may saturate the catalyst oxygen storage. After the deceleration event, when the engine cylinders are again combusting air and fuel, NOx engine emissions may not be reduced in the catalyst because the catalyst is saturated with oxygen and there may not be stored HC and CO to reduce the NOx engine emissions.
To address this problem in prior approaches, the engine was operated open loop (e.g. no feedback control) rich of stoichiometry for a predetermined time after the deceleration event to de-saturate the catalyst oxygen storage. After a predetermined time the engine controller would then commence feedback control with the exhaust gas oxygen sensor tracking stoichiometry as described above.
The inventors herein have recognized numerous issues with the above approach. If the open loop operation rich of stoichiometry fully depleted the oxygen stored during a previous DFSO event, then during subsequent feedback control the rich transient that generates HC and CO engine emissions may not be oxidized across the catalyst surface because there is no stored oxygen available. Further, if not enough oxygen is depleted during the open loop operation, then during subsequent feedback control NOx generated during a lean transient may not be reduced across the catalyst surface.
The inventors herein have solved these issues by open loop operation of the engine air/fuel ratio rich of stoichiometry for a predetermined time after the deceleration event, followed by feedback control of the air/fuel ratio on average around a value rich of stoichiometry for a preselected time after the predetermined time, and feedback control of the air/fuel ratio gradually returning to an average around stoichiometry after the preselected time. Gradually returning the air/fuel ratio to an average around stoichiometry may be based on a number of engine cycles, for example 3 to 10 engine cycles, or may be within an allotted time span. The preselected time may be related to the predetermined time. In this manner, the stored oxygen after a deceleration event is not fully depleted so the HC and CO will be oxidized during a rich transient caused by the air/fuel control. And, the feedback control at a value rich of stoichiometry will reduce the generation of lean transients and resulting NOx generation that may not otherwise have been reduced. Feedback controlling the air/fuel ratio on average around a value rich of stoichiometry may comprise gradually changing the value to stoichiometry during the preselected time. In another aspect of the disclosure, after the deceleration event, the engine air/fuel ratio is operated open loop at a constant air/fuel ratio rich of stoichiometry for a predetermined time before the exhaust gas oxygen sensor positioned downstream of the catalyst switches to a rich state and before oxygen stored in the catalyst is fully depleted, subsequently the engine air/fuel ratio is feedback controlled in response to the exhaust gas oxygen sensor to average around a value rich of stoichiometry for a preselected time after the predetermined time, and the air/fuel ratio is then feedback controlled responsive to the downstream exhaust gas oxygen sensor to average around stoichiometry after the preselected time. During the feedback control following the open loop control, the sensor switch point is set at a value rich of stoichiometry. Thus, lean transients that would have been generated at a stoichiometric switch point are reduced. The NOx that would have been generated during lean transients which might have passed through a catalyst containing too much stored oxygen is avoided. By the time normal feedback control around a stoichiometric sensor set point is commenced, the catalyst will have a proper balance of stored oxygen and NOx breakthrough will not occur.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.